1. Field of the Invention
The present invention relates to a contact probe for a testing head having vertical probes effective to test a plurality of semiconductor integrated electronic devices comprising a plurality of so-called contact pads and more particularly to a contact probe wherein a plurality of probes are inserted in guide holes realized in respective plate-like holders, or dies, the probe comprising a rod-shaped body equipped at an end with at least a contact tip effective to ensure the mechanical and electrical contact with a corresponding contact pad of an integrated electronic device to be tested.
2. Description of the Related Art
As it is well known, a testing head is essentially a device effective to electrically connect a plurality of contact pads of a semiconductor integrated electronic device with corresponding channels of a testing machine performing the test thereof.
The test performed on integrated electronic devices provides to detect and isolate defective devices already in the manufacturing step. Generally, testing heads are thus used to electrically test electronic devices integrated on semiconductor or silicon wafer before cutting and assembling them inside a chip package.
A testing head having vertical probes comprises at least a pair of parallel plates or plate-like holders located apart from each other in order to keep an air gap, as well as a plurality of suitable mobile contact elements.
Each plate, called die in the relevant technical field and in the following description, is equipped with a respective plurality of through guide holes, each hole of a plate corresponding to a hole of the other plate wherein a respective contact element or contact probe, as said element will be called in the following description and in the subsequent claims, is slidingly engaged and guided. Contact probes are generally composed of wires made of special alloys with good electrical and mechanical properties.
The good electrical connection between the probes of the testing head and the contact pads of an integrated electronic device to be tested is ensured by urging each contact probe onto the respective contact pad, mobile contact probes elastically bending in the air gap between the two dies.
These testing heads are generally called “vertical probe”.
In essence, known testing heads have an air gap wherein a probe bending occurs, this bending being helped through a convenient configuration of the probes themselves or of the dies thereof, as schematically shown in FIG. 1.
In this FIG. 1 a testing head 1 comprises at least an upper die 2 and a lower die 3, having respective upper 4 and lower 5 through guide holes wherein a contact probe 6 is slidingly engaged.
The contact probe 6 has at least a contact end or tip 7. In particular, the contact tip 7 is mechanically contacted with a contact pad 8 of an integrated electronic device to be tested, said integrated electronic device meanwhile electrically contacting a testing equipment (not shown) of which this testing head is a terminal element.
Upper 2 and lower 3 dies are spaced by an air gap 9 allowing contact probes 6 to be deformed or sloped during the testing head normal operation, i.e. when this testing head comes into contact with the integrated electronic device to be tested. Moreover upper 4 and lower 5 guide holes are sized in order to guide the contact probe 6.
FIG. 1 shows a testing head 1 with unblocked probes, i.e. being capable of sliding in respective upper 4 and lower 5 guide holes, associated with a micro-contact strip or space transformer, schematically indicated with 10.
In this case, contact probes 6 have a further contact tip towards a plurality of contact pads 11 of the space transformer 10, the good electrical contact between the probes and the space transformer 10 being ensured similarly to the contact with the integrated electronic device to be tested by urging the probes 6 onto the contact pads 11 of the space transformer 10.
In particular, according to the technology known as Cobra, contact pads 6 have a pre-deformed configuration with an offset d between the end in contact with the contact pads 11 of the space transformer 10 and the contact tip 7 on the contact pads 8 of the integrated electronic device to be tested, as schematically shown in FIG. 2.
The pre-deformed configuration, also in case the testing head 1 is not in contact with the integrated electronic device to be tested, favors the correct bending of the probe 6 during the operation thereof, i.e. during the contact with the integrated electronic device to be tested.
Moreover, a thin and flexible insulating material film 12, generally realized in polyimide, is interposed between the upper die 2 and the lower die 3, able to keep the upper end of the contact probes 6 in place during the assembly step.
In particular, the assembly step of a testing head 1 realized according to the Cobra technology is particularly delicate. It comprises the following steps:                each contact probe 6 is inserted from the corresponding side to the contact tip 7 in a hole in the lower die 3, as schematically shown in FIG. 3;        the other end of contact probes 6 is softly forced into the conveniently drilled flexible material film 12, so that it is held by this material film 12 without risking to exit therefrom, as schematically shown in FIG. 4, and        once all contact probes 6 are inserted in the flexible material film 12 as described, the upper die 2 is applied, centering with great skill all contact probes 6 in the corresponding holes realized in the upper die 2, as schematically shown in FIG. 5.        
This assembly mode required by the probes realized according to the Cobra technology is very long, besides being risky for possible probe deformations and very unstable up to the locking with the upper die 2.
Moreover, there is the risk of mechanical interference between the flexible material film 12 and contact probes 6 during the normal operation of the testing head 1 since this film 12, once its task of keeping the probe 6 end in place during the assembly step is completed, nevertheless remains trapped inside the testing head 1 and it can cause serious problems to the sliding of the probes themselves, mainly for large-sized testing heads, with a high number of close probes.
It is also known to realize testing heads by using the so-called “shifted plate” technology, schematically shown in FIG. 6, the elements being structurally and functionally identical to the testing head 1 of FIG. 1 having been indicated with the same numeral references.
In this case, contact probes 6 are not pre-formed, but only realized in a straight form, with circular cross section being constant for the whole length thereof and generally pointed at the ends.
In order to achieve the correct operation of contact probes 6, the upper 2 and lower 3 dies are conveniently shifted one another to allow probes 6 to bend preferentially in a same direction.
The assembly of probes 6 in testing heads realized according to the shifted plate technology is very simple and fast and it does not require the use of any flexible material film. In particular, it is sufficient to align the upper die 2 with the lower die 3 in order to align also the corresponding guide holes 4 and 5, to insert then the contact probes 6 in the guide holes 4 and 5, to shift the dies therebetween by a convenient quantity then blocking them in position.
However, also this technology has some drawbacks, and in particular:                it is difficult to keep contact probes 6 within their housing, i.e. inside the die guide holes. In fact, despite the relative shift between the upper 2 and lower 3 dies, causing a friction between the contact probes 6 and the corresponding guide holes 4 and 5, this friction is not always sufficient to keep probes in place.        
In particular, the risk of exit of contact probes 6 is much higher during the maintenance and cleaning operations of the testing head 1, operations which are generally performed with air blows or ultrasounds and which thus create mechanical stresses on contact probes 6, favoring the exit thereof from guide holes.
The distance between two adjacent probes of the testing head 1 is limited because of the circular cross section of the wire realizing the contact probes 6.
In particular, testing heads have intrinsic distance limits between two adjacent probes, and thus between the centers of two contact pads of the integrated electronic device to be tested, known in this field as “pitch”. In particular, the minimum “pitch” value depends on the probe geometrical configuration and size. In order to avoid the contact between adjacent probes, the testing head 1 must satisfy the following relation:P>ØF+G1
being:
P the pitch value of the device to be tested, i.e. the distance between the centers of two adjacent contact pads;
ØF the diameter of the contact probes 6; and
G1 the safety distance between adjacent contact probes 6.
The condition G1=0, i.e. the annulment of the safety distance, corresponds to the probe collision.
In the case of circular probes, the minimum pitch P1 is given by the probe diameter ØF corresponding to the diameter of the guide holes increased by the thickness G1 of the wall separating two adjacent holes, as schematically shown in FIG. 7.
The need to keep a minimum distance pitch value between the probes is thus in contrast with the present market need, which pushes to design denser and denser devices and thus requires testing heads with a higher and higher number of contact probes for testing these devices.